1. Technical Field
The present invention relates to a vehicle occupant restraint system. In particular, the present invention relates to an energy management device for seat belt webbing used in a seat belt system.
2. Description of the Prior Art
Seat belt systems are commonly used to restrain vehicle occupants. A typical seat belt system for restraining a vehicle occupant includes a length of belt webbing extensible about the occupant and having opposite ends anchored to the vehicle. In the event of sudden deceleration such as occurs in a vehicle collision, the occupant tends to move forward relative to the vehicle and is restrained by the belt webbing.
The vehicle occupant can experience a sudden load if the occupant's forward movement is restrained by an inextensible length of belt webbing. To alleviate this sudden load, it is known to stitch together portions of the belt webbing with a series of spaced apart tear stitches to shorten the belt webbing. When the occupant engages the belt webbing with sufficient force, the load on the belt webbing tears the stitching. The belt webbing portions separate and the belt webbing extends slightly. The occupant's movement is restrained at a reduced rate over an increased period of time. This can reduce the abruptness of the load experienced by the vehicle occupant.
As each one of the tear stitches tears, the load on the occupant changes abruptly. Further, each one of a series of tear stitches can tear with a different amount of resistance than other ones of the series of tear stitches. As a result, energy management characteristics of a seat belt system including tear stitches can be uneven.